I understand you are particularly interested in the involvement of the Plains Indians in the Civil War. I find it fascinating as well and am interested in any reliable documentation that can be found. They are certainly not representative of the thousands of Native Americans who participated in the war but do fit the Hollywood stereotypes people expect to see.

The best photo of a Confederate Indian in the Trans-Mississippi is that of Lt. Pleasant Porter, 1st Creek Mtd Vols, which is on the cover of G.W. Grayson's memior "A Creek Warrior for the Confederacy" (There's a much better reproduction of the photo in the book but I couldn't find it online). Grayson was a captain in the 1st Creek Mtd Vols and later principal chief of the Creek (Muscogee) Nation. As I recall, Grayson said Porter's clothing was the same as his own.

http://www.amazon.com/gp/customer-media/product-gallery/0806121033/ref=cm_ciu_pdp_images_all

One of my favorite photos is that of Sgt Zeke Proctor, Co. L, "the artillery company", 3rd Indian Home Guard (US), "the Pin Regiment". I found a copy online about the middle of the page at:

http://www.accessgenealogy.com/native/proctor/ezekiel-proctor.htm